285 research outputs found
On the stellar populations in NGC 185 and NGC 205, and the nuclear star cluster in NGC 205 from Hubble Space Telescope observations
[Abridged] We present a first detailed analysis of resolved stellar
populations in the dwarf galaxies NGC 185 and NGC 205 based on archival V- and
I-band WFPC2 pointings. For NGC 185 we deduce that star formation was probably
still active about 4 x 10^8 yr ago. Key abundance-related results are: (1) We
identify ancient stars with [Fe/H] <~ -1.5 dex by a well-defined horizontal
branch (HB). (2) We find a prominent RGB/ faint-AGB clump/ bump- like feature
with the same mean V-band magnitude as the HB, within uncertainties; from a
comparison with theory, ancient stars have [Fe/H] ~ -1.5 dex, with a higher
abundance level for intermediate-age stars. (3) From colour information we
infer that the median [Fe/H] > -1.11 +/- 0.08 dex for ancient stars. For NGC
205, we record (m-M)o = 24.76 +/- 0.1 mag, based on the RGB I-band tip
magnitude method. We find that stars were probably still forming less than 3 x
10^8 yr ago, which is compatible with star formation triggered by an
interaction with M31. Key abundance-related results are: (1) The RGB/ faint-AGB
is significantly skewed to redder values than that of a control field in the
outskirts of M31; it probably results from a relatively narrow metallicity and
or age range for a significant fraction of the dwarf's stars. (2) For ancient
stars we infer from colour information that the median [Fe/H] > -1.06+/-0.04
dex. We briefly compare the stellar populations of NGC 205, NGC 185 and NGC
147.
Finally, we find an apparent blue excess in the outer region of the nuclear
star cluster in NGC 205. It is as compact as a typical galactic globular
cluster, but is quite bright (10^6 L_solar,R); and by matching its blue colour
with models, its stellar population is young, up to a few times 10^8 yr old.Comment: To appear in the May edition of the Astronomical Journal. Some
figures have been degraded in quality for the purpose of submissio
Stellar Kinematics and Structural Properties of Virgo Cluster Dwarf Early-Type Galaxies from the SMAKCED Project. I. Kinematically Decoupled Cores and Implications for Infallen Groups in Clusters
We present evidence for kinematically decoupled cores (KDCs) in two dwarf
early-type (dE) galaxies in the Virgo cluster, VCC 1183 and VCC 1453, studied
as part of the SMAKCED stellar absorption-line spectroscopy and imaging survey.
These KDCs have radii of 1.8'' (0.14 kpc) and 4.2'' (0.33 kpc), respectively.
Each of these KDCs is distinct from the main body of its host galaxy in two
ways: (1) inverted sense of rotation; and (2) younger (and possibly more
metal-rich) stellar population. The observed stellar population differences are
probably associated with the KDC, although we cannot rule out the possibility
of intrinsic radial gradients in the host galaxy. We describe a statistical
analysis method to detect, quantify the significance of, and characterize KDCs
in long-slit rotation curve data. We apply this method to the two dE galaxies
presented in this paper and to five other dEs for which KDCs have been reported
in the literature. Among these seven dEs, there are four significant KDC
detections, two marginal KDC detections, and one dE with an unusual central
kinematic anomaly that may be an asymmetric KDC.The frequency of occurence of
KDCs and their properties provide important constraints on the formation
history of their host galaxies. We discuss different formation scenarios for
these KDCs in cluster environments and find that dwarf-dwarf wet mergers or gas
accretion can explain the properties of these KDCs. Both of these mechanisms
require that the progenitor had a close companion with a low relative velocity.
This suggests that KDCs were formed in galaxy pairs residing in a poor group
environment or in isolation whose subsequent infall into the cluster quenched
star formation.Comment: 14 pages, accepted for publication in Ap
Honesty mediates the relationship between serotonin and reaction to unfairness
How does one deal with unfair behaviors? This subject has long been investigated by various disciplines including philosophy, psychology, economics, and biology. However, our reactions to unfairness differ from one individual to another. Experimental economics studies using the ultimatum game (UG), in which players must decide whether to accept or reject fair or unfair offers, have also shown that there are substantial individual differences in reaction to unfairness. However, little is known about psychological as well as neurobiological mechanisms of this observation. We combined a molecular imaging technique, an economics game, and a personality inventory to elucidate the neurobiological mechanism of heterogeneous reactions to unfairness. Contrary to the common belief that aggressive personalities (impulsivity or hostility) are related to the high rejection rate of unfair offers in UG, we found that individuals with apparently peaceful personalities (straightforwardness and trust) rejected more often and were engaged in personally costly forms of retaliation. Furthermore, individuals with a low level of serotonin transporters in the dorsal raphe nucleus (DRN) are honest and trustful, and thus cannot tolerate unfairness, being candid in expressing their frustrations. In other words, higher central serotonin transmission might allow us to behave adroitly and opportunistically, being good at playing games while pursuing self-interest. We provide unique neurobiological evidence to account for individual differences of reaction to unfairness
Dissecting early-type dwarf galaxies into their multiple components
Early-type dwarf galaxies, once believed to be simple systems, have recently
been shown to exhibit an intriguing diversity in structure and stellar content.
To analyze this further, we started the SMAKCED project, and obtained deep
H-band images for 101 early-type dwarf galaxies in the Virgo cluster in a
brightness range of -19 \leq M_r \leq -16 mag, typically reaching a
signal-to-noise of 1 per pixel of \sim0.25" at surface brightnesses \sim22.5
mag/arcsec^2 in the H-band. Here we present the first results of decomposing
their two-dimensional light distributions. This is the first study dedicated to
early-type dwarf galaxies using the two-dimensional multi-component
decomposition approach, which has been proven to be important for giant
galaxies. Armed with this new technique, we find more structural components
than previous studies: only a quarter of the galaxies fall into the simplest
group, namely those represented by a single S\'ersic function, optionally with
a nucleus. Furthermore, we find a bar fraction of 18%. We detect also a similar
fraction of lenses which appear as shallow structures with sharp outer edges.
Galaxies with bars and lenses are found to be more concentrated towards the
Virgo galaxy center than the other sample galaxies.Comment: Accepted by ApJL, 6 pages, 4 figures, 1 tabl
The Effect of a Single Supernova Explosion on the Cuspy Density Profile of a Small-Mass Dark Matter Halo
Some observations of galaxies, and in particular dwarf galaxies, indicate a
presence of cored density profiles in apparent contradiction with cusp profiles
predicted by dark matter N-body simulations. We constructed an analytical
model, using particle distribution functions (DFs), to show how a supernova
(SN) explosion can transform a cusp density profile in a small-mass dark matter
halo into a cored one. Considering the fact that a SN efficiently removes
matter from the centre of the first haloes, we study the effect of mass removal
through a SN perturbation in the DFs. We found that the transformation from a
cusp into a cored profile is present even for changes as small as 0.5% of the
total energy of the halo, that can be produced by the expulsion of matter
caused by a single SN explosion.Comment: 6 pages, 4 figures, accepted for publication in MNRA
Abundance Gradients and the Formation of the Milky Way
In this paper we adopt a chemical evolution model, which is an improved
version of the Chiappini, Matteucci and Gratton (1997) model, assuming two main
accretion episodes for the formation of the Galaxy. The present model takes
into account in more detail than previously the halo density distribution and
explores the effects of a threshold density in the star formation process,
during both the halo and disk phases. In the comparison between model
predictions and available data, we have focused our attention on abundance
gradients as well as gas, stellar and star formation rate distributions along
the disk. We suggest that the mechanism for the formation of the halo leaves
detectable imprints on the chemical properties of the outer regions of the
disk, whereas the evolution of the halo and the inner disk are almost
completely disentangled. This is due to the fact that the halo and disk
densities are comparable at large Galactocentric distances and therefore the
gas lost from the halo can substantially contribute to building up the outer
disk. We also show that the existence of a threshold density for the star
formation rate, both in the halo and disk phase, is necessary to reproduce the
majority of observational data in the solar vicinity and in the whole disk.
Moreover, we predict that the abundance gradients along the Galactic disk must
have increased with time and that the average [alpha/Fe] ratio in stars (halo
plus disk) slightly decrease going from 4 to 10 Kpcs from the Galactic center.
We also show that the same ratios increase substantially towards the outermost
disk regions and the expected scatter in the stellar ages decreases, because
the outermost regions are dominated by halo stars.Comment: 41 pages (including the figures), To be published in Ap
WFPC2 Observations of Leo A: A Predominantly Young Galaxy within the Local Group
The unprecedented detail of the WFPC2 colour-magnitude diagrams of the
resolved stellar population of Leo A presented here allows us to determine a
new distance and an accurate star formation history for this extremely
metal-poor Local Group dwarf irregular galaxy. From the position of the red
clump, the helium-burning blue loops and the tip of the red giant branch, we
obtain a distance modulus, m-M=24.2+/-0.2, or 690 +/- 60 kpc, which places Leo
A firmly within the Local Group. Our interpretation of these features in the
WFPC2 CMDs at this new distance based upon extremely low metallicity (Z=0.0004)
theoretical stellar evolution models suggests that this galaxy is predominantly
young, i.e. <2 Gyr old. A major episode of star formation 900 - 1500 Gyr ago
can explain the red clump luminosity and also fits in with our interpretation
of the number of anomalous Cepheid variable stars seen in this galaxy. We
cannot rule out the presence of an older, underlying globular cluster age
stellar population with these data. However, using the currently available
stellar evolution models, it would appear that such an older population is
limited to no more than 10% of the total star formation to have occured in this
galaxy. Leo A provides a nearby laboratory for studying young metal poor stars
and investigations of metal-poor galaxy evolution, such as is supposed to occur
for larger systems at intermediate and high redshifts.Comment: 64 pages, 18 figures, 4 tables accepted for publication in the
Astronomical Journal (Sept. 98
The physics of dipolar bosonic quantum gases
This article reviews the recent theoretical and experimental advances in the
study of ultracold gases made of bosonic particles interacting via the
long-range, anisotropic dipole-dipole interaction, in addition to the
short-range and isotropic contact interaction usually at work in ultracold
gases. The specific properties emerging from the dipolar interaction are
emphasized, from the mean-field regime valid for dilute Bose-Einstein
condensates, to the strongly correlated regimes reached for dipolar bosons in
optical lattices.Comment: Review article, 71 pages, 35 figures, 350 references. Submitted to
Reports on Progress in Physic
ViCTORIA project: MeerKAT HI observations of the ram pressure stripped galaxy NGC 4523
We present the first results of a 21 cm HI line pilot observation carried out
with MeerKAT in preparation for the ViCTORIA project, an untargeted survey of
the Virgo galaxy cluster. The extraordinary quality of the data in terms of
sensitivity and angular resolution (rms~0.65 mJy beam^-1 at ~27"x39" and 11
km/s resolution) allowed us to detect an extended (~10 kpc projected length)
low column density (N(HI) < 2.5x10^20 cm^-2) HI gas tail associated with the
dwarf irregular galaxy NGC4523 at the northern edge of the cluster. The
morphology of the tail and of the stellar disc suggest that the galaxy is
suffering a hydrodynamic interaction with the surrounding hot intracluster
medium (ICM; ram pressure stripping). The orientation of the trailing tail, the
gradient in the HI gas column density at the interface between the cold ISM and
the hot ICM, the velocity of the galaxy with respect to that of the cluster,
and its position indicate that NGC4523 is infalling for the first time into
Virgo from the NNW background of the cluster. Using a grid of hydrodynamic
simulations we derive the impact parameters with the surrounding ICM, and
estimate that the galaxy will be at pericentre (D~500-600 kpc) in ~1 Gyr, where
ram pressure stripping will be able to remove most, if not all, of its gas. The
galaxy is located on the star formation main sequence when its star formation
rate is derived using Halpha images obtained during the VESTIGE survey,
suggesting that NGC4523 is only at the beginning of its interaction with the
surrounding environment. A few HII regions are detected in the Halpha images
within the HI gas tail outside the stellar disc. Their ages, derived by
comparing their Halpha, FUV, NUV, and optical colours with the predictions of
SED fitting models, are <30 Myr, and suggest that these HII regions have formed
within the stripped gas.Comment: Accepted for publication in A&
Fitting the integrated Spectral Energy Distributions of Galaxies
Fitting the spectral energy distributions (SEDs) of galaxies is an almost
universally used technique that has matured significantly in the last decade.
Model predictions and fitting procedures have improved significantly over this
time, attempting to keep up with the vastly increased volume and quality of
available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of
multiwavelength data sets, and the methods used to fit model SEDs to observed
galaxy data sets. We touch upon the achievements and challenges in the major
ingredients of SED fitting, with a special emphasis on describing the interplay
between the quality of the available data, the quality of the available models,
and the best fitting technique to use in order to obtain a realistic
measurement as well as realistic uncertainties. We conclude that SED fitting
can be used effectively to derive a range of physical properties of galaxies,
such as redshift, stellar masses, star formation rates, dust masses, and
metallicities, with care taken not to over-interpret the available data. Yet
there still exist many issues such as estimating the age of the oldest stars in
a galaxy, finer details ofdust properties and dust-star geometry, and the
influences of poorly understood, luminous stellar types and phases. The
challenge for the coming years will be to improve both the models and the
observational data sets to resolve these uncertainties. The present review will
be made available on an interactive, moderated web page (sedfitting.org), where
the community can access and change the text. The intention is to expand the
text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics &
Space Scienc
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